1. Field of the Invention
The invention relates to a process and a circuit layout for the simulation of test stand inertia moments wherein the test stand and the specimen represent an n-mass oscillator, the masses of which are elastically joined together.
2. Background of the Art
In order to obtain realistic acceleration behavior, test stands for automotive components (for example, engines, gears, axles) are at present for the most part expected to allow dynamic tests in which an essential criterion is the simulation of the actual moment of inertia of the vehicle.
The moment of inertia of the vehicle may be simulated mechanically by appropriate disk flywheels mounted on the test stand. However, this method, which is frequently used on brake test benches, is very uncomfortable and does not allow variations of the moment of inertia during operation. If for example the test specimen is an internal combustion engine, it moves against a rigidly coupled disk flywheel, while in the case of the vehicle, the mass of the vehicle is joined elastically by means of the elasticities of the tires, axles, drive gears and shaft with the engine. The specimen is therefore loaded on a test stand of this type in a dynamically incorrect manner.
Test stands are being constructed further on which the moment of inertia is simulated by an electric machine. Different control and regulating algorithms are known, whereby the vehicle inertia moment desired is to be simulated.
The paper by Hans-Jurgen von Thun, having a German language title which translates as "Brake Test Stand with Disk Flywheel Simulation", published in the German journal Elektro-Anzeiger, No. 34, (1981), pages 49 to 53, describes a process for mass simulation, wherein it is assumed that the moment produced by the test specimen may be measured without reaction. In the case of many types of test stands, in particular with engine test stands, this is possible only at great expense or not at all, so that for such test stands the process described in the paper is not applicable.
To simulate the dynamic behavior of internal combustion engines, Hans-Jurgen von Thun described a process providing for electric inertia moment simulation, which is multiplied by the angular acceleration of the test stand in a paper entitled "Simulation of the Dynamic Performance of the Engine By an Electric Motor Serving as Test Rig Drive", Conference Proceedings of Electronics for National Security, pages 397 to 411, Sept. 27-29, 1983, Brussels, Belgium. The simulation moment formed in this manner is communicated to the electric test stand machine as the reference value. In this process the rpm signal of the test stand must be differentiated. The simulation circuit produces a retroaction because of the use of the angular acceleration; it possesses stability and dynamic restrictions which become apparent particularly in the case of simulated inertia moments deviating strongly upward and downward from the mechanically present moment of inertia.
West German Offenlegungsschrift No. 32 25 035 describes an apparatus for the testing of a torque generator. The shaft is coupled in normal operation to a load charged with inertial moment. The process upon which this apparatus is based, again does not operate, as stated, with signals free of retroaction. As proof of the signals free of retroaction is presented in the form of equations, which do not contain the dynamic components of the measuring shaft signal (page 11, bottom), it is stated that the mass of the specimen is connected rigidly with the electric drive machine of the test stand. This, however, is not true for highly dynamic systems. Furthermore, the process described therein does not represent anything novel in relation to dynamics and stability. Merely different signals obtained in a more cumbersome manner are used for simulation than those of the process described in the preceding paragraph. The process according to West German Offenlegungsschrift No. 32 25 035 is thus subject to the same stability and dynamic restrictions as the process described in the preceding paragraph.
It is an object of the invention to provide a process and a circuit layout for the simulation of dynamic torque components, in particular the simulation of test stand inertia moments, whereby the quality of simulation is improved. In the process the elastic joining to the specimen is to be taken into consideration as it exists in the actual vehicle. This is especially true in view of the newer test methods wherein the effect of the dynamics of the entire vehicle on the test specimen is to be observed. A rigid coupling of the inertia moments to be simulated to the mechanically present masses of the test stand and the specimen is disadvantageous, as there are simply no determined parameters for new natural frequencies and the attenuation which should be taken into consideration in the case of altered spring-mass systems. There exists therefore no degree of freedom in the vicinity of the limits of stability. The invention is intended to remedy this situation.
This object is attained according to the invention for a process and a circuit layout for the simulation of test stand inertia moments, in which the test stand and the test specimen represent an n-mass oscillator; the masses whereof are coupled with each other mechanically (spring constant C.sub.n-1 and attenuation measures d.sub.n) by means of analog and/or digital electronic functional elements; m further masses with spring constants; and attenuations are simulated electrically in a manner such that the structure corresponds to the system of differential equations of the (n+m) mass system; and at least one moment controlled electric machine is used as a transmission element between the electronic functional elements and the mechanical masses.
The process shall be described herein by means of the example of an internal combustion engine test stand, which is to simulate the entire gear train of the vehicle with respect to the torsional movement. Obviously, the process is suitable also for other types of test stands where inertia moments are to be simulated.